There have been two problems with polyethylene coated liquid containing containers. These problems are the carton leaking and the side walls of the carton bulging over time. The first of these, the leaking, is created by cracks or tears in the carton which allows the liquid to leak out. The second of these, the bulging, is caused by the paperboard between the plastic liners taking up the liquid causing it to lose strength.
One of the standard methods of overcoming this problem was to use thicker layers of polyethylene on the paperboard. This was both to reduce the pinholes in the plastic which allow liquid to enter the paperboard and to prevent tearing or splitting of the polyethylene coating.
Over the years there have been a number of other solutions proposed.
Dobbins, et al., U.S. Pat. No. 3,207,410 discloses a multipiece container in which the fiber side wall is coated with a thin film 42 of thermoplastic resin, preferably polyethylene. When the container is assembled, the interior is flushed and given a thin wax coating 50. The wax coating drains down the side wall before solidifying and provides a liquid proof end seam of improved strength between the side wall and base.
Gordy, U.S. Pat. No. 3,305,383 discloses leakage along score lines at or adjacent to the bottom of the container and suggests impregnating a flat carton blank at certain confined areas with a normally solid hydrophobic material, such as various waxes. The critical areas are the areas along the bend or score lines at the bottom of the carton and extending up the sides. The wax or water resistant material is applied to the side of the flat blank which will become the inside of the carton. The outside of the blank may also be treated. The substrate may then be heated to facilitate penetration of the water resistant material into the stock. The substrate is then coated with the polyethylene thermoplastic resin.
McNair, Jr., et al., U.S. Pat. No. 3,365,111 discloses applying a hot melt after the container is formed. The hot melt may be applied either to the center of the bottom closure structure as in FIG. 5A or over the entire area of the bottom closure as in FIG. 5B.
Thompson, et al., U.S. Pat. Nos. 3,421,678 and 3,482,278 are exemplary of several that disclose the application of a variable thickness polyethylene coating to the paperboard substrate prior to the formation of a carton. This results in a coating thinner than normal in the upper portion of the carton and thicker than normal in the lower portion of the carton. FIG. 3 of U.S. Pat. No. 3,421,678 shows paperboard having the variable thickness interior polyethylene film in cross section.
The Brownlee, et al. U.S. Pat. Nos. 3,913,825 and 3,913,826 disclose other methods of improving the bottom of the paperboard container to make it leak proof. U.S. Pat. No. 3,913,826 discloses a continuous thermoplastic band 92 along the bottom edge of the blank which melts and seals the edges of the bottom closure together when the container is formed. The band 92 may be replaced by discontinuous bands 93-96 which seal the major leakage channel when the container is erected and heat sealed. The thermoplastic must adhere to polyethylene and have a melt index of 3.0-300. Stein Hall LHM 597 is recommended.
U.S. Pat. No. 3,913,826 disloses thermoplastic patches 86, 87, 90 and 91 which form melted patch 99 when the container is erected and heat sealed. Again the thermoplastic must adhere to polyethylene and have a melt index of 3.0-300. Dupont Surlyn AD 8109 and polyethylene are recommended.
Geisler, U.S. Pat. No. 2,676,745 discloses treating the score lines of a corrugated container with a solution of latex and sodium silicate to toughen them.
Kachurchak, U.S. Pat. No. 3,355,081 discloses placing a plastic strip on the interior of the container so that when the container is slit with a knife, the knife will enter the strip and not cut the material contained within the container. The strip is resilient and has high strength. It may be nylon, polyethylene, polypropylene or any other molded or extruded plastic.
British Pat. No. 948,279 discloses a cardboard blank in which the interior is coated with polyethylene or polypropylene. The coating is thicker in the areas in which the blank is to be heat sealed than in the areas where it is not to be heat sealed.
Hawkins, U.S. Pat. No. 3,245,601 discloses a patterned coating on the substrate.
Moors, et al. U.S. Pat. No. 3,137,436 discloses a milk carton in which the vertical score lines are covered and reinforced with a varnish. The varnish may be a nitrocellulose varnish, an acrylic varnish or a polyvinyl chloride varnish. The container is formed and then the interior walls are coated with wax.
In another development in the early 1960s, a milk carton was reinforced along the score lines with an Elvax material having a viscosity of from 75,000 to 100,000 cp. The entire board was then coated with an Elvax composition having a viscosity of less than 75,000 cp.
In each of the above constructions the paperboard itself is coated with the reinforcing material and then the additional coating, usually of wax, is placed over the reinforcing material.